Arguably, the most well-known rotating speaker system in the audio effects field is referred to as the “Leslie” speaker, named after its inventor, Donald Leslie. One version of the Leslie speaker has two rotating horns, one in front of a stationary high-frequency speaker and one in front of a stationary low-frequency speaker, all in a single cabinet. The rotation of the horns produces a tremolo effect (amplitude modulation) and a variation in pitch due to the Doppler effect (frequency modulation). As stated in Leslie's U.S. Pat. No. 2,489,653, “it is not necessary that the horns from the high and low frequency speakers rotate in synchronism; in fact, best results are frequently obtained by rotating the speakers at different speeds and in opposite directions.” Leslie's patent does not disclose synchronizing the absolute angular positions of the two horns as they rotate.
There have been many variations of the Leslie speaker concept over the years, each creating a variation of the tremolo effect. However, none have achieved the acoustic effects that are possible only through control of the absolute angular positions of two or more rotating speakers (or rotating horns or baffles) in a multi-rotor speaker array.
What is needed, therefore, is a multi-rotor speaker array in which the absolute angular position of one rotating speaker in relation to the absolute angular position of another rotating speaker is known and controlled.